A lumped element circuit device is one that can be described by a simple linear equation during operation at high frequencies, generally 1 GHz or higher. Lumped element circuit devices are used at high frequencies in circuits such as high-pass filters, including frequencies in the microwave region where electronic circuit elements designed for adequate performance at lower frequencies may exhibit severely degraded operating characteristics. In general, to be considered a lumped element electronic component, the component must be dimensioned much smaller than a quarter wavelength of the intended maximum operating frequency, and in fact no feature of the component's structure should exceed 1/10 of a wavelength at the maximum operating frequency.
One common method for fabricating sufficiently miniaturized high-frequency capacitors and related circuits is with the use of LTCC materials as the body and dielectric of the capacitor, and metal conductive layers applied with screen printing or other deposition techniques. The details of these fabrication techniques are well known in the art, and an example of this type of capacitor can be seen in U.S. Pat. No. 5,144,526 entitled LOW TEMPERATURE CO-FIRED CERAMIC STRUCTURE CONTAINING BURIED CAPACITORS.
An example of an LTCC-implemented high-frequency filter can be seen in U.S. Pat. No. 6,816,032 entitled LAMINATED LOW-PROFILE DUAL FILTER MODULE FOR TELECOMMUNICATIONS DEVICES AND METHOD THEREFOR. The capacitors implemented in this module are very simple, with one plate of each capacitor formed by an associated ground plane.
As operating frequencies increase, so does the effect of the equivalent series inductance (ESL) of the capacitor. The voltage change ΔV can be expressed asΔV=ESL*di/dt where di/dt expresses the change in current with time. Thus, as the di/dt increases as frequency increases, the effect of the ESL becomes more of a problem to device function. The ESL, also called parasitic inductance, causes the capacitor to deviate from theoretical ideal operating conditions and results in an insertion loss when the capacitor is used in a high frequency circuit. Such insertion losses can seriously compromise circuit performance.
Some LTCC capacitors have been designed to mitigate the parasitic inductance present at high frequencies. A typical approach to solving the parasitic inductance problem is taught in U.S. Pat. No. 7,054,134 entitled STACKED CAPACITOR. Multiple internally deposited conductors are stacked alternately between dielectric layers and attached to multiple extraction electrodes which serve as terminals. The geometry of partially overlapping extraction electrodes of opposite polarities serves to generate oppositely flowing currents between any two conductors, which separated by thin dielectric layers, serves to cancel out a portion of the magnetic flux generated by the current in each conductor, thus reducing the capacitor's ESL.
A drawback to the structure taught by the '134 patent is the need to fabricate a very complex multilayered structure, where the addition of each dielectric layer, internal conductor and extraction electrode contributes increased cost and the chance for fabrication errors causing decreased performance and rejected components. A further drawback is the need to make electrical contact to the multiplicity of electrodes when the capacitor is assembled into the final circuit, where each additional solder connection also represents an additional chance for failure and an additional possible limitation on frequency response. In general, the fewer external connections that are needed between elements in a high frequency circuit, the higher the frequencies that the circuit can respond to without unacceptable losses. Additionally, the complexity of the structure taught by the '134 patent raises significant barriers to economical incorporation of this type of capacitor into an LTCC-implemented circuit such as a filter.
A significant improvement over the existing art would be a lumped capacitor that could be economically and reliably fabricated for use in high-frequency circuits such as filters designed to be produced with standard LTCC fabrication techniques, using a minimum number of dielectric layers, conductors and terminals, having a minimum of parasitic inductance and thus yielding a broad frequency performance and low insertion loss close to the theoretical ideal.